1. Field of Invention
The present invention relates to an atomizing device comprising a fine tube as fluid path and its method of manufacture for providing a repeatable performance in terms of droplet size and spatial droplet distribution. The invention is particularly suitable for coating medical devices and for creating fine aerosols.
2. Background of the Invention
Atomizing devices comprising a fine liquid tube as fluid line are used in various applications, such as medical nebulizers, chemical analysis of liquid samples and coating devices to atomize small amounts of liquids.
FIG. 1 depicts an enlarged view of the front section of an exemplary atomizer known by the prior art comprising an inner liquid delivery tube and a support member to secure the liquid delivery tube. A cap may be provided at the exit end of the liquid tube to form an annular gap 106 between the inner liquid delivery tube and the cap surrounding the liquid tube. The support member includes a central bore, having an internal diameter larger than the outside diameter of tube 104, for aligning the liquid tube. Additional points for alignment of the liquid tube (not shown) may be provided by central bores disposed within the support member having slightly larger diameters than the outside diameter of the tube.
The liquid tube may be additionally secured by various mechanisms, such as compression fittings as described in U.S. Pat. App. No. US2005/0029442, collet type connections as described in U.S. Pat. No. 6,337,480 or brace like support structure as provided by U.S. Pat. Nos. 5,868,322 and 6,032,876.
Optimum atomization and particle transport efficiencies generally depends on the spatial characteristics of the spray plume and on the droplet size which, in turn, depends on the shape of the atomizer tip and/or on the roundness and concentricity of the annular gap. This is particularly true, when an atomizing gas is provided through a comparatively small annular gap.
However, with current atomizers there is relatively poor control over the concentricity between the tube support member assembly and the cap for the atomizing gas, resulting in a misalignment of liquid tube relative to cap. A stable and secure support of the liquid tube may not be ensured because the tube is generally not sufficiently supported proximate to the liquid exit. Mechanisms used to support the liquid tube are often connections that don't ensure precise and repeatable positioning of the liquid tube in relation to the cap due to assembly tolerances. For example, tolerances between the outside diameter of tube 104 and inside diameter of bore 103, and tolerances between the location shoulder and cap, as shown in FIG. 1, may lead to an increased error in concentricity between the liquid tube and cap. In U.S. Pat. Nos. 5,868,322 and 6,032,876 an atomizing device is provided having an outer tube comprising a brace-like support structure for mechanically securing the inner liquid tube. However, precise concentricity of the liquid tube and outer tube may not be ensured due to assembly tolerances and possible imperfections of the inner and/or outer tube. In addition, the brace-like support structure comprises gas channels, which may cause constrictions within the flow path. Thus, turbulence in the gas flow may be produced resulting in an unstable flow field and inconsistent spray performance. Furthermore, the gas channels, which are disposed along the micro tube, constitute limitations for generating a flow field with an angular momentum, which may be desirable to improve the atomization process.
Another drawback of conventional atomizers, which usually comprise premanufacturered liquid tubes, are imperfections of the tube in terms of roundness and surface quality as well as manufacturing limitations. Due to the relatively small outside diameter and long length of such liquid tubes it may be difficult or even not possible to compensate such imperfections by machining the tip region. To overcome given quality limitations of prefabricated liquid tubes and maximize concentricity it is desirable to machine or even shape the tip of the liquid tube, which is particularly beneficial in electrostatic spraying applications. In addition, there is a risk of misalignment of the liquid tube in relation to the cap when disassembling and reassembling the tube support member assembly during cleaning and maintenance operations, which may result in poor spray performance.
It has been furthermore found that when pneumatic assisted atomizers are used, comprising a very fine liquid tube that is not sufficiently stabilized towards the atomizing end, the spray performance may change during the same spray run. Gas stream 105, as depicted in FIG. 1, which exits the annular gap and surrounds the liquid tube, may have an impact on position and alignment of the liquid tube in relation to the gas orifice. Depending on operating conditions and atomizer configuration, the gas flow may cause the liquid tube to oscillate and change its position during operation, resulting in an inconsistent spray performance of the atomizer over time.
Imperfections of the tube tip and/or in the annular region directly translate into an inhomogeneous spray pattern, a relatively wide size range of droplets and increased droplet sizes. In addition, the shape and surface quality of the atomizing end at the liquid exit may influence the droplet break up and may result in poor efficiency of the atomization process, particularly in the case of electrostatic atomization. The spray performance of pneumatic atomizers, in terms of symmetric spatial particle distribution and tight droplet size distribution, is closely related to the roundness and concentricity of the annular gap. Any imperfection and eccentricity between the axes of the liquid delivery tube and the cap can cause the flow of the atomizing gas to be cylindrically asymmetric with respect to the axis of the liquid exiting from the liquid delivery tube. Hence, inhomogeneous gas velocities within the annular gap will lead to nebulization by the atomizing gas that is different on different sides of the spray plume.
Poor spray stability and droplets that are too large and polydisperse in size may result in poor reproducibility and often poor stability during operation which, in turn, may lead to coating defects or reduced sample analysis efficiency.